Serna lets you open documents with a single click thanks to document templates that associate all necessary metadata with a certain document type.
On opening a document, Serna automatically finds a document template which matches the document, and applies the appropriate schema, stylesheet, etc.
Serna has document templates for both DITA 1.0 and DITA 1.1 documents. However, all DITA documents are opened by default as DITA 1.1 documents.
The thing is that while DITA 1.1 documents have the xsi:noNamespaceSchemaLocation attribute that allows Serna to determine the DITA version correctly, DITA 1.0 documents don't have such an attribute and are impossible to be recognized as 1.0. To open a DITA 1.0 document with a DITA 1.0 template, follow the instructions.